2D Materials for Gas Sensing

Transcription

1 2D Materials for Gas Sensing S. Guo, A. Rani, and M.E. Zaghloul Department of Electrical and Computer Engineering The George Washington University, Washington DC 20052

2 Outline Background Structures of 2D Material for Gas Sensing TMDS Material Examples of 2D material for gas sensing and Results Conclusion and Future Work Acknowledgement 9/14/2017 M.E.Zaghloul 3

3 Background Recently 2D materials have been used for gas sensing because of the atomic-thin layered structure, large surface-to-volume ratio, and large adsorbing capacity of gas molecules and strong surface activities. Recently layered inorganic material analogues of graphene, such as Transition Metal Dichalcogenides (TMDs) were developed. These material include MoS 2, WS 2, MoSe 2, WSe 2, ReS 2, and ReSe 2, as well as layered metal oxides (MoO 3 and SnO 2 ). Unlike graphene, TMDs monolayers have the potential to exist in more than one possible crystal structure. This implies : Semiconducting (2H) phase Semi-metal (1T ) phase Tunable Band gap, high sensitivity for varieties of chemicals. Thickness dependent Physical and Chemical properties. 9/14/2017 M.E.Zaghloul 4

4 Gas Sensing Mechanism The Gas sensing mechanism is based on transfer of charges, in which the sensing material acts as absorber or donors of charges. The charge transfer between the gas molecules and the sensing material will cause changes in the sensing material properties. 9/14/2017 M.E.Zaghloul 5

5 Semiconductor gas sensing mechanism Classification Oxidising Gases Reducing Gases N-type R increase R decrease P-type R decrease R increase 9/14/2017 M.E.Zaghloul 6

6 Gas Sensing Device (FET) structure The gas sensor works by bridging the two electrodes (source and drain) with sensing materials and passing current through them. Gas detection can be realized by monitoring the current change upon exposure to the target gas environment under a constant voltage. Structure of the FET Device For conductance type gas sensor, both high sensitivity and fast recovery rate are desirable. 9/14/2017 M.E.Zaghloul 7

7 Various Types of 2D Gas Sensing devices There are several structures of 2D material Gas sensing devices, examples of such devices are: Chemiresistor Sensors Field Effect Transistors FETs Schottky diodes Conductometric sensors Surface Acoustic Wave (SAW) sensors 9/14/2017 M.E.Zaghloul 8

8 Performance Parameters The performance parameters for gas sensing are characterized by: Sensor Response Time Selectivity Sensitivity Stability Recovery Time 9/14/2017 M.E.Zaghloul 9

9 Performance Parameters Factors The Gas sensor Performance is influenced by several parameters: - The sensing material type and dimension - Humidity - Temperature - Gas flow rate There are several approaches to improve the performances of the sensor such as using programming temperature, the use of UV light into the sensor, and the use Nano Particles as catalyst to improve absorption and thus improve selectivity. 9/14/2017 M.E.Zaghloul 10

10 Our Group Research on Gas Sensing We are studying gas sensing properties for two TMDs materials: MoSe 2 MoTe 2 9/14/2017 M.E.Zaghloul 11

11 Gas sensors using MoSe 2 Molybdenum diselenide (MOSe 2 ) An inorganic compound, its structure is similar to MoS 2 Single Crystal layers of MoSe 2 and flakes are exfoliated from bulk Crystal Electron mobility of 2D MoSe 2 is higher than MoS 2 It has direct band gap 9/14/2017 M.E.Zaghloul 12

12 9/14/2017 M.E.Zaghloul 13

13 Transfer material Gold Assisted Exfoliated Ref : Sujay B. Desai et al.; Gold-Mediated Exfoliation of Ultralarge Optoelectronically-Perfect Monolayers; Adv. Mater. 2016, 28, /14/2017 M.E.Zaghloul 14

14 Flakes Transferred Optical image of transferred flake 9/14/2017 M.E.Zaghloul 15

15 MoSe2 Characterization using Raman and PL Raman Spectra PL 9/14/2017 M.E.Zaghloul 16

16 Mask and Optical image of the Contact Pad device Etched channel 9/14/2017 M.E.Zaghloul 17

17 FET characteristics (n-type) Transfer Characteristic Output Characteristic 9/14/2017 M.E.Zaghloul 18

18 Gas sensing set up Structure 9/14/2017 M.E.Zaghloul 19

19 Gas response (20ppm NO2) Gas on Gas on off off Sun Young Choi et al.; Effect of Nb Doping on Chemical Sensing Performance of Two-Dimensional Layered MoSe2; ACS Appl. Mater. Interfaces 2017, 9, In comparison, MoSe 2 always have large recovery time which we need to find a way to improve. Our device can reach 20% response for 20ppm NO2 and shows great sensitivity. 9/14/2017 M.E.Zaghloul 20

20 Challenges with the measured Data Large recovery time 400s How to improve the sensitivity in the range of 100ppb - 10ppm How to improve the selectivity 9/14/2017 M.E.Zaghloul 21

21 Work we are doing to improve the results 1. Heat assist to help recovery 2. FET structure/uv assist to help improve sensitivity 3. Metal nanoparticles decoration to help improve selectivity 9/14/2017 M.E.Zaghloul 22

22 MoTe 2 Molybdenum ditelluride (MoTe 2 ) is an especially attractive 2D material because it exists in two stable polymorphs. MoTe 2 1. Semiconducting hexagonal (2H) and 2. Semi-metallic monoclinic (1T ) Two phases can be reversibly transformed by altering MoTe 2 crystal growth conditions or by a post-growth thermal treatment. 9/14/2017 M.E.Zaghloul 23

23 Intensity (a. u.) MoTe 2 2-D Material Material Characterization of 2H-MoTe 2 MoTe 2 crystals, slow cooling MoTe 2 crystals grown from: 1T' poly-mote 2 2H poly-mote 2 (008) 2H Cooling rate defines a crystal structure of MoTe 2 single crystals, regardless of the initial (004) crystal phase of the poly-mote 2. (002) (006) (degrees) X-Ray diffraction patterns were obtained using milled MoTe 2 powders and platelets. 9/14/2017 M.E.Zaghloul 24

24 Material Characterization of 2H-MoTe 2 Raman spectra were acquired from the 2H- MoTe 2 flakes exfoliated onto SiO 2 /Si substrates. The spectra for the 2H flakes, exhibit an out-of-plane A g mode around 170 cm -1, an in-plane E 2g mode around 235 cm -1 and an out-ofplane B 2g mode at 289 cm -1. All lines are in excellent agreement with Raman studies reported in the literature [14, 15], which confirms the phase and quality of the flakes. 9/14/2017 M.E.Zaghloul 25

25 Electrical Characterization Optical Image Current Voltage graph Schematic MoTe 2 Au/Ti SiO 2 /Si Annealing was done at 350 ᵒC Ag 9/14/2017 M.E.Zaghloul 26

26 Electrical Characterization Output Characteristic Transfer Characteristic P-type 9/14/2017 M.E.Zaghloul 27

27 Packaging of MoTe 2 For gas sensor Three different thickness MoTe 2 FET are packaged on above chip-career for gas-sensing measurement. 9/14/2017 M.E.Zaghloul 28

28 Approaches to improve the Gas-sensing performance 1. Incorporation of UV light into gas sensors. 2. High sensitivity can be achieved by good Schottky barrier modulation of 2D material and metal electrode junction. 3. Surface functionalization of the 2D channel between source and drain electrodes. Example : Pt nanoparticles on MoS 2 thin films. 4. Decoration of 2D material with metal oxides. Example : MoS 2 /SnO 2 channel between source and drain electrode. 5. Annealing the 2D sensing material at proper temperature. 9/14/2017 M.E.Zaghloul 29

29 Summary It is an important to develop Very sensitive and Very selective gas sensing device for many applications such as health, food, agriculture, Two dimensional (2D) material is showing great potential in gas sensing with high sensitivity due to their high surface-to-volume ratio and promising semiconductor properties. Our Goal is to develop portable wearable gas sensing device 9/14/2017 M.E.Zaghloul 30

30 Acknowledgement NIST Collaborators Dr. Albert Davydov : Supervisor Dr. Sergiy Kylyuk : Growth of material Dr. Ryan Beams : Raman Spectroscopy Dr. R. Debnath : Electrical Characterization Irian Kalish : X-Ray Diffraction 9/14/2017 M.E.Zaghloul 31